Byron Swift has given directions for how he has been monitoring the Cliff Swallow colony at/under the Brunswick Potomac River Bridge [Maryland Route 17 to Virginia State Route 287].  I haven't checked it out myself, so i can't provide exact details.  When I do – today perhaps - I will give an updated report to MDOsprey.  In the meantime, in case anyone happens to be in the area and wants to take a chance on the incomplete (?) directions:

Follow the thread below, but basically he seems to have been peering at them from the VA bridge end, a little distance down a lane leading from 287 opposite (?) the gas station property. 

We are still awaiting word from SHA regarding the impact of the bridge work on the colony.  MOS’ Kurt Schwarz has been the primary point of contact with them.  He has been much more patient with them than I would have been, but I understand his approach and it may lead to a MOS/SHA collaboration to prevent future such impacts on nesting colonies...Jim


From Byron (edited slightly):
I just look at them from a few feet from the side of the bridge on the VA side, I imagine you could check also from the MD side - the swallows are more or less in the middle. It may be hard to see all clearly if the tarps are covering part of the site, but if you see them repeatedly coming in and out that is a pretty good sign things are active. Might take a few minutes.

and

I assume the bridge goes east/west across the river - the birds are nesting mid-way along the bridge in about the middle of the bridge on both the upriver (north) side and the downriver (south) side, and accessing their nests through a gap in the curtain. When I was there last week (7/12/10), the tarpaulin is like a curtain drawn across most of the bridge, but it is parted in the middle and that is where the birds were coming in, on both sides.

Jim Speicher
Jspeicher—at—niaid.nih.gov
Rockville @work :(
Broad Run_S. FRED Co @home :)